The present invention relates to a bearing device using a dynamic-pressure bearing where a lubricant is sealed to the bearing surface and also relates to a spindle motor provided with the bearing device.
In most of the spindle motors used in information equipment, ball bearings have been employed for supporting a rotating body. However, the ball bearing has limitations on high-accurate rotation and high-speed rotation and has become an obstacle to the high-accurate and high-speed operation of information equipment. When, on the other hand, a rotating body is supported by a sliding bearing, very high accurate rotation is possible because the rotating body can be supported by a film of dynamic-pressure oil with non-contact, and the sliding bearing is suitable for high-speed rotation.
A bearing device adopting the sliding bearing is disclosed, for example, in Japanese Patent Unexamined Publication No. 61-201916. In this publication, axial and radial gaps are provided between a rotational shaft and a housing as a means of preventing axial and radial vibrations, and magnetic fluid is sealed in the gaps so that the rotational shaft can be rotatably supported. In addition, seal devices are provided on the opposite ends of the housing, and step-shaped bearing-force generating members are provided on the surface and the opposite ends of the rotational shaft. With the dynamic-pressure effect caused by rotation, accuracy of rotation is stably maintained while suppressing vibration.
Although a sliding bearing is not employed, in Japanese Utility Model Unexamined Publication No. 3-117120 there is disclosed a method of lubricating a radial bearing. A rotational shaft is made hollow and one end thereof is immersed in an oil collecting portion provided in the lower portion of the radial bearing. Oil is drawn up by the pumping effect caused due to rotation and is supplied to holes opened at positions corresponding to upper and lower radial bearings, whereby the lubrication of the radial bearings disposed on upper and lower sides of the rotational shaft is performed.
In addition, in Japanese Patent Unexamined Publication No. 6-187720 there is disclosed a structure where a pre-load is applied to a spindle in the radial direction by a magnetic pre-load device disposed eccentrically with respect to the spindle to enhance the rigidity of the bearing. Axial and radial flow passages are provided in the spindle. One end of the axial flow passage is communicated with the surface of a thrust bearing, and the radial flow passage is communicated with the oil chamber and the space between upper and lower radial bearings. A lubricant is supplied from one end of the axial flow passage to the bearing portion and then sealed with a plug.
When the spindle is supported by the sliding bearing, the spindle needs to be positioned in the radial and axial directions and also radial and thrust bearings are needed as a means of preventing vibration, and a lubricant is sealed by using a seal device in the radial and thrust bearings.
On the other hand, when the spindle is assembled or the lubricant is sealed, there is the problem that air remains on the bearing portion and bubbles flow to the sliding surface of the bearing. In this case, the oil film breaks on the bearing surface, the rigidity is remarkably reduced, and unstable vibration occurs. If temperature rises, bubbles will grow and this tendency will be further accelerated.
In the aforementioned technique disclosed in Japanese Patent Unexamined Publication No. 61-201916, the rigidity of the oil film is increased by the dynamic-pressure effect of the step-shaped bearing-force generating groove to enhance accuracy of rotation, however, a means of removing bubbles from the bearing during rotation is not provided. Therefore, if air particles remain unremoved in the bearing when a lubricant is sealed, the air particles will grow into bubbles. As a result, there is the possibility that the bubbles flow to the lubrication surface, impede the dynamic-pressure effect of the bearing-force generating groove, reduce the rigidity of the lubricant, and give rise to unstable vibration.
In the aforementioned technique disclosed in Japanese Utility Model Unexamined Publication No. 3-117120, a lubricating oil is drawn from the end of the hollow shaft by the pumping effect caused due to rotation and is supplied to the ball bearing, and the problem of the dynamic pressure reduction in the bearing portion caused by bubbles does not exist. Therefore, this technique pays no regard to the means for removing bubbles.
In the aforementioned technique disclosed in Japanese Patent Unexamined Publication No. 6-187720, axial and radial flow passages are provided in a spindle, and a lubricant is supplied from one end of the spindle to radial and thrust bearings and then sealed with a plug. The other end of the spindle is disposed in contact with the thrust bearing, so when the spindle is rotated, a pumping effect of sucking the lubricant from the other end of the spindle into the axial flow passage cannot be expected. Therefore, the axial and radial flow passages do not serve as a means of removing bubbles.